Solar Variability/Climate Link Research Comes in from the Cold

An important new study has been published in the Earth and Space Science journal of the AGU, which establishes a close correlation between solar activity (namely, the end of solar cycles) and the transition from prevailing El Nino to La Nina conditions in the Pacific. Says one of the authors:

“Energy from the Sun is the major driver of our entire Earth system and makes life on Earth possible,” said Scott McIntosh, a scientist at the National Center for Atmospheric Research (NCAR) and co-author of the paper. “Even so, the scientific community has been unclear on the role that solar variability plays in influencing weather and climate events here on Earth. This study shows there’s reason to believe it absolutely does and why the connection may have been missed in the past.”

The researchers looked at the 22 year Hale Cycle and established a statistically significant correlation between the well defined end of the Hale Cycle and sea surface temperatures in the central Pacific:

The 22-year cycle begins when oppositely charged magnetic bands that wrap the Sun appear near the star’s polar latitudes, according to their recent studies. Over the cycle, these bands migrate toward the equator—causing sunspots to appear as they travel across the mid-latitudes. The cycle ends when the bands meet in the middle, mutually annihilating one another in what the research team calls a terminator event. These terminators provide precise guideposts for the end of one cycle and the beginning of the next.

The researchers imposed these terminator events over sea surface temperatures in the tropical Pacific stretching back to 1960. They found that the five terminator events that occurred between that time and 2010-11 all coincided with a flip from an El Nino (when sea surface temperatures are warmer than average) to a La Nina (when the sea surface temperatures are cooler than average). The end of the most recent solar cycle—which is unfolding now—is also coinciding with the beginning of a La Nina event.

It appears to be very unlikely that the correlation is purely coincidental, with now six terminator events being simultaneous with a switch from El Nino to Lan Nina conditions:

In fact, the researchers did a number of statistical analyses to determine the likelihood that the correlation was just a fluke. They found there was only a 1 in 5,000 chance or less (depending on the statistical test) that all five terminator events included in the study would randomly coincide with the flip in ocean temperatures. Now that a sixth terminator event—and the corresponding start of a new solar cycle in 2020—has also coincided with an La Nina event, the chance of a random occurrence is even more remote, the authors said.

The study did not attempt to establish a causal mechanism, but critically it has established beyond all reasonable doubt that there must exist some causal relationship between major fluctuations in the solar magnetic field and climate variability on earth.

In the paper itself, the authors state:

 A forecast of the Sun’s global behavior places the next solar cycle termination in mid‐2020; should a major oceanic swing follow [it has], then the challenge becomes: when does correlation become causation and how does the process work?

In the previous section, we have made use of a modified Superposed Epoch Analysis (mSEA) to investigate the relationships between solar activity measures and variability in a standard measure of the variability in the Earth’s largest ocean—the Pacific. We have observed that this mSEA method brackets solar activity and correspondingly systematic transitions from warm‐to‐cool Pacific conditions around abrupt changes in solar activity we have labeled termination points. These termination points mark the transition from one solar activity (sunspot) cycle to the next following the cancellation or annihilation of the previous cycle’s magnetic flux at the solar equator—the end of Hale magnetic cycles.

Correlation does not imply causation; however, the recurrent nature of the ONI signal in the terminator fiducial would appear to indicate a strong physical connection between the two systems. Appendix B discusses three statistical Monte Carlo tests that show the chances of these events lining up for five cycles are remote: in summary we may reject the null hypothesis of random cooccurrences with a confidence level p < 3.4 × 10−3

What’s interesting is that the authors identify the past several decades as a ‘default El Nino like state’ when cloud cover in the Western Pacific has been depleted, coincident with a weakened Pacific Walker circulation and strengthened Brewer-Dobson circulation. During this period, they argue that ENSO has been uniquely sensitive to variations in solar activity:

Thus, over the past several decades the cloud pattern in the western Pacific has adopted an almost El Niño‐like default state, consistent with an observed eastward shift in precipitation in the tropical Pacific and weakening of the Walker circulation over the last century (Deser et al., 2004; Vecchi & Soden, 2007a), and which has been tied, via simple thermodynamics, to a warmer atmosphere.

Thus, it is entirely plausible that since changes in the (upper) atmosphere brought on by a strengthened Brewer‐Dobson circulation, weakened Pacific Walker circulation, and less cloudy Western Pacific, enables the relatively constant terminator‐driven changes to have sufficient “impact” to flip the system from El Niño to La Niña, independent of the actual mechanism that couples solar changes to clouds and ENSO.

The 2020 termination of the last Hale cycle, marked by the end of SC24 and beginning of SC25 is, according to Valentina Zharkhova, the beginning of a Maunder-like Minimum which will last from 2020-2053. If, as she suggests, global surface temperatures decline during this period, then we might expect the relationship between Terminator events and the switch from El Nino to La Nina to become less pronounced. The current progression of the Pacific to a La Nina may in fact be the beginning of a phase change from an ‘El Nino-like default state’ to a La Nina-like default state where, ironically, solar activity has less of an influence on central Pacific ocean surface temperatures. We shall see. All I can at this present time is that it’s extremely cold here in England at the start of April – it’s been perishing most days since January – and it certainly feels like a Maunder Minimum! Two very warm days at the end of March doesn’t quite convince me that global warming has not deserted the UK!


  1. Willis is not happy with the way they established a connection between terminator events and the transition to La Nina. He thinks they did not robustly define the transitions. Worth noting in that respect that the authors say:

    “we focus here on the National Oceanic and Atmospheric Administration (NOAA)‐generated “Oceanic Niño Index” (ONI). Note that in this paper, we are not trying to explore every bump and wiggle in the ONI—our primary focus are the “decadal‐scale” large transitions from El Niño (hot mid‐Pacific) to La Niña (cold mid‐Pacific), the signature “El Niño Events” like that in 1997–1998 (Trenberth & Stepaniak, 2001). A visual comparison between the termination points in the solar data panels and the ENSO/ONI record would appear to indicate that there is a possible relationship between the termination points and positive‐to‐negative swings in the ONI record. Just as we not trying to explore every bump and wiggle, we do not try to predict or understand the magnitude of that swing, just that it exists.”

    So it might be reasonably argued that their ‘transitions’ are not precisely defined, but that is perhaps an inevitability when looking at phenomena like ENSO – there is no sharply defined delineation between the two states, just a gradual progression between ‘La Nina like’ and ‘El Nino like’ with clearly defined major events like the El Ninos of 1997/98 and 2015/16 and the La Nina of 1998/99.


  2. NOAA seems to have considerably complicated the definition of El Niño, and Willis admits he has his own concept of it.
    Being Australian, I tend to just look at the SOI, but also pay attention to “cloudiness at the dateline” graph from the BOM, which seems to me to indicate best what is actually going on in the Nino area.
    It’s interesting that OLR at the dateline varies by >100w/m^2 between the two states. The SOI had collapsed last time I looked but OLR was still high. Given that, I wouldn’t be surprised to see a second La Niña event follow this last one, as sometimes happens.


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